Selective developmental defects of cord blood antigen-presenting cell subsets

Defective antigen-presenting cell (APC) function has been hypothesized to contribute to increased infection susceptibility in newborns. We used multiparameter flow cytometry to characterize APC subsets in adult peripheral blood (APB) and cord blood (CB). APB had a higher proportion of CD11c+ dendritic cells (DC), whereas CB mainly contained CD123+ DC. APB was enriched in CD16+CD11c+ DC subset, whereas CD34+CD11c-CD123lo cells were prominent in CB. Lipopolysaccharide (LPS)-induced tumor necrosis factor (TNF)-alpha production was dampened in myeloid DC and monocytes from CB, whereas IL-1alpha production was not different. The reduction in TNF-alpha response did not appear to result from reduced surface detection of LPS, because CD14, toll-like receptor (TLR)-4 and TLR-2 levels were not reduced in CB APC compared with APB cells. Also, there was no correlation between TLR-2 or TLR-4 levels and TNF-alpha production in myeloid DC and monocytes. CB monocytes had lower surface HLA-DR immediately ex vivo. Both APB and CB monocytes upregulated HLA-DR after incubation, but an additional LPS-induced increase in HLA-DR was suggested only in APB monocytes. APB monocytes also showed a greater LPS-induced increase in CD40 expression. Together, our data show significant, selective differences in circulating APC between neonates and adults.     

Enveloped virus but not bacteria block IL-13 responses in human cord blood T cells in vitro

Infections that occur early in life may have a beneficial effect on the immune system and thereby reduce the risk of allergen sensitization and/or allergic disease. It is not yet clear to what extent specific virus and/or bacteria can mediate this effect. The purpose of this study was to assess the role of virus and bacteria in CD4(+) T cell-derived cytokine production in newborns. We compared the effects of five bacteria (Staphlococcus aureus, Escherichia coli, Clostridium difficile, Lactobacillus rhamnosus and Bifidobacterium bifidus) and seven virus (adenovirus, coronavirus, cytomegalovirus, herpes simplex virus, influenza virus, morbillivirus and poliovirus) on the Th1/Th2 cytokine production in mixed lymphocyte reactions using CD4(+) T cells from cord blood cocultured with allogenic myeloid or plasmacytoid dendritic cells. When comparing the baseline cytokine production prior to microbial stimulation, we observed that cord plasmacytoid DC were stronger inducers of Th2 cytokines (IL-5 and IL-13) compared with cord myeloid DC and to adult DC. When adding microbes to these cultures, bacteria and virus differed in two major respects; Firstly, all enveloped viruses, but none of the bacteria, blocked Th2 (IL-13) production by cord CD4(+) cells. Secondly, all Gram-positive bacteria, but none of the virus, induced IL-12p40 responses, but the IL-12p40 responses did not affect Th1 cytokine production (IFN-γ). Instead, Th1 responses were correlated with the capacity to induce IFN-α secretion, which in cord cells were induced by S. aureus and influenza virus alone. These data imply that enveloped virus can deviate Th2 responses in human cord T cells.     

Differential toll-like receptor expression after ex vivo lipopolysaccharide exposure in patients with sepsis and following surgical stress

INTRODUCTION: Monocytes from septic patients have a reduced capacity to respond to lipopolysaccharide (LPS). We examined whether the same response occurred after surgical injury, and whether this reduced activity was associated with differential monocyte toll-like receptor (TLR) expression. MATERIALS AND METHODS: Peripheral blood mononuclear cells (PBMCs) were isolated from septic patients, patients undergoing surgery, and healthy volunteers. Cells were stimulated ex vivo with LPS (1 microg/ml) and stained for CD14, CD16, TLR-2, TLR-4, and HLA-DR surface expression. RESULTS: TLR-2 and -4 expressions were significantly increased in monocytes from both septic and surgical patients. While ex vivo LPS-stimulation significantly increased TNFalpha and IL-1beta production in PBMCs from surgical patients, LPS-stimulation decreased IL-1beta production from septic patients as compared to surgical and control patients. Ex vivo LPS-stimulation induced TLR-4 upregulation in monocytes from both surgical and control patients, but not from septic patients. HLA-DR expression in CD14+CD16+ monocytes was reduced only in septic patients. CONCLUSIONS: PBMCs from septic patients, but not following surgical injury, have a reduced capacity to respond to a secondary inflammatory signal, but this defect is not associated with reduced TLR-4 or CD14 expression.     

Human Langerhans' cells and dermal-type dendritic cells generated from CD34 stem cells express different toll-like receptors and secrete different cytokines in response to toll-like receptor ligands

Langerhans' cells (LC) and dermal dendritic cells (dDC) are located in the superficial and deeper layers of the skin respectively and represent the main dendritic cell (DC) populations of the skin. LC-like and dDC-like DC can be generated from CD34 stem cells and this system is widely used as a model for investigating these cells and in therapeutic vaccination. Here we report toll-like receptor (TLR) expression in human LC and dDC derived from CD34 stem cells. In vitro-generated DC expressed TLR-1, 2, 4, 6, 8 and 10. LC, but not dDC, expressed TLR-5, whereas only dDC expressed TLR-3. Maturation of LC was mediated by TLR-2, 4 and 5 ligands, but not by a TLR-3 ligand. dDC maturation was induced by TLR-3 and -4, but not with TLR-5 ligand and only weakly by a TLR-2 ligand. Stimulated LC secreted interleukin (IL)-1beta, low levels of tumour necrosis factor-alpha (TNF-alpha) and IL-8, but not IL-6 or IL-10. dDC secreted TNF-alpha, IL-6, IL-8 and IL-10, but little IL-1beta. IL-12p70 was not produced by ligand-stimulated dDC or LC, but was secreted by monocyte-derived DC (mdDC) stimulated with lipopolysaccharide (LPS). Thus, in vitro-generated LC and dDC detect different pathogen-associated molecules and show different cytokine-secretion profiles in response to TLR ligands.     

Synthetic mycoplasma-derived lipopeptide MALP-2 induces maturation and function of dendritic cells

Dendritic cells (DC) modulate immune responses depending on the nature of the antigens. Receptors capable of discriminating these antigens on the basis of the pathogen-associated molecular patterns (PAMP) belong to the Toll-like receptor (TLR) family. The macrophage-activating lipopeptide 2 kDa (MALP-2), a synthetic lipopeptide derived from Mycoplasma fermentans, signals through TLR-2 and TLR-6. The aim of this study was to examine whether MALP-2 can modulate the functional properties of human monocyte-derived DC. The effects of this treatment were compared to those of the TLR-4 agonist lipopolysaccharide (LPS). To ensure clinical applicability, DC were generated under serum-free conditions. MALP-2 and LPS stimulation induced the expression of CD83 and increased the expressions of CD80, CD86, HLA-ABC and CD40. Furthermore, both substances decreased the endocytotic capacity of DC and induced the release of bioactive TNF-alpha and IL-10, whereas LPS additionally increased IL-12 release. Pretreatment with both substances boosted the allostimulatory capacity of DC. In a coculture with autologous lymphocytes, either MALP-2 or LPS pretreated DC induced a marked proliferation of lymphocytes, but only DC prestimulated with MALP-2 activated lymphocytes to produce the cytokines IL-4, IL-5 and IFN-gamma. No polarisation of lymphocytes into T-helper (Th)1 or Th2 was detected. These data indicate that MALP-2 is a potential candidate to modulate DC for clinical applications.     

CD16+ human monocyte-derived dendritic cells matured with different and unrelated stimuli promote similar allogeneic Th2 responses: regulation by pro- and anti-inflammatory cytokines

We previously demonstrated that tumor necrosis factor (TNF)-alpha-matured CD16- and CD16+ human monocyte-derived dendritic cells (16-mDC and 16+mDC) differentially stimulate naive CD4+ lymphocytes by inducing Th1- and Th2-like responses, respectively. Here, we further characterized the role of different DC maturation factors on Th polarization. Immature 16+mDC and 16-mDC (iDC) obtained by culture of purified monocytes with GM-CSF and IL-4 were maturated with (i) Toll-like receptor (TLR) ligands [lipopolysaccharide (LPS)], (ii) lymphocyte-derived (soluble CD40 ligand, IFN-gamma) and (iii) endogenous inflammatory stimuli [TNF-alpha, prostaglandin (PG)E2]. After activation with these stimuli, DC secrete IL-12 only in presence of LPS, and 16+mDC produced lower amounts of IL-12 and IL-10 than 16-mDC. Allogeneic CD4+CD45RO- lymphocytes co-cultured with 16+mDC secreted higher levels of IL-4 and IL-10 than those co-cultured with 16-mDC, regardless of the maturation stimuli. Results were similar when DC were activated with TLR-2 or TLR-3 ligands. The higher induction of IL-4 by 16+mDC was primarily dependent on IL-12, IL-4 and IL-10. IFN-gamma production by CD4+ T cells was similar with all the conditions except with LPS-16+mDC, which induced reduced amounts of this cytokine. Those differences were totally eliminated by neutralization of IL-12, IL-4 or IL-10. Finally, 16-mDC could reverse the Th2 phenotype of already committed lymphocytes toward a Th1 pattern in short-term cultures, whereas 16+mDC had less ability to skew this phenotype. These results indicate that 16+mDC elicit superior Th2 responses independently of the maturation factors that they received, and suggest that they could represent an important population of regulatory DC.     

TLR-mediated induction of negative regulatory ligands on dendritic cells

Dendritic cells (DCs) shape T-cell response patterns and determine early, intermediate, and late outcomes of immune recognition events. They either facilitate immunostimulation or induce tolerance, possibly determined by initial DC activation signals, such as binding Toll-like receptor (TLR) ligands. Here, we report that DC stimulation through the TLR3 ligand dsRNA [poly(I:C)] limits CD4 T-cell proliferation, curtailing adaptive immune responses. CD4+ T cells instructed by either lipopolysaccharide (LPS) or poly(I:C)-conditioned DCs promptly upregulated the activation marker CD69. Whereas LPS-pretreated DCs subsequently sustained T-cell clonal expansion, proliferation of CD4+ T cells exposed to poly(I:C)-pretreated DCs was markedly suppressed. This proliferative defect required DC-T cell contact, was independent of IFN-alpha, and was overcome by exogenous IL-2, indicating T-cell anergy. Coinciding with the downregulation, CD4+ T cells expressed the inhibitory receptor PD-1. Antibodies blocking the PD-1 ligand PD-L1 restored proliferation. dsRNA-stimulated DCs preferentially induced PD-L1, whereas poly(I:C) and LPS both upregulated the costimulatory molecule CD86 to a comparable extent. Poly(dA-dT), a ligand targeting the cytoplasmic RNA helicase pattern-recognition pathway, failed to selectively induce PD-L1 upregulation, assigning this effect to the TLR3 pathway. Poly(I:C)-conditioned DCs promoted accumulation of phosphorylated SHP-2, the intracellular phosphatase mediating PD-1 inhibitory effects. The ability of dsRNA to bias DC differentiation toward providing inhibitory signals to interacting CD4+ T cells may be instrumental in viral immune evasion. Conversely, TLR3 ligands may have therapeutic value in silencing pathogenic immune responses.     

Critical role of protein kinase C epsilon for lipopolysaccharide-induced IL-12 synthesis in monocyte-derived dendritic cells

In the present study we have investigated the potential involvement of protein kinase C (PKC) in the maturation of human dendritic cells (DC) by bacterial lipopolysaccharide (LPS). LPS stimulation of DC derived from human monocytes resulted in PKC phosphorylation. Inhibition of PKC activation using bisindolylmaleimide (Bis), a pan-PKC inhibitor, was associated with a dose-dependent decrease of LPS-induced IL-12 production. In contrast, up-regulation of MHC class II, CD80 and CD86 was not altered. Consistent with the diminished IL-12 synthesis, DC stimulated with LPS in presence of Bis were deficient in the induction of IFN-gamma production by allogeneic CD4+ T cells. Furthermore, we found that PKC inhibition impaired LPS-induced I kappa B-alpha degradation and subsequent nuclear factor (NF)-kappa B activation in DC. LPS resulted in the phosphorylation of conventional alpha/beta and novel epsilon PKC isoforms in DC. Inhibition of LPS-induced PKC activity using pseudosubstrate peptides specific for PKC isoforms established that PKC epsilon but not PKC alpha/beta was involved in the production of IL-12 and TNF-alpha. Overall, these data provide evidence that PKC inhibition impairs LPS signaling in DC and identify PKC epsilon as a potential target for the inhibition of Toll-like receptor-4-mediated, IL-12-dependent Th1 type responses.     

Multiple Sclerosis: IFN-β Induces CD123+BDCA2– Dendritic Cells that Produce IL-6 and IL-10 and have No Enhanced Type I Interferon Production

IFN-β, an approved drug for multiple sclerosis (MS), acts on dendritic cell (DC) by suppressing their production of IL-12p40 and increasing IL-10. This results in Th2-biased immune responses. The nature of IFN-β-modulated DC remains elusive. Previously, we observed that IFN-β dose dependently induces expression of CD123, i.e. a classical marker for plasmacytoid DC, on human blood monocyte-derived myeloid DC. Such IFN-β-modulated DC produce predominantly IL-10 but are IL-12 deficient, with potent Th2 promotion. In the present study, we further characterize IFN-β-modulated DC by using recently identified blood DC antigens (BDCA) and investigate their ability to produce Type I IFN in response to virus stimulation. We show that IFN-β induces development of CD123⁺ DC from human blood monocytes, which coexpress BDCA4⁺ but are negative for BDCA2^–, a specific marker for plasmacytoid DC. Such IFN-β-modulated DC produce large amounts of IL-6 and IL-10, but no IL-12p40 and have no enhanced IFN-β and IFN-β production. The findings indicate that IFN-β-modulated DC represent a myeloid DC subset with diminished CD11c, BDCA-1 and CD1a expression, having potent Th2-promoting function but lacking antiviral capacity.     

Maturation of murine bone marrow-derived dendritic cells with poly(I:C) produces altered TLR-9 expression and response to CpG DNA

Although poly(I:C) and LPS induced differential dendritic cell (DC) cytokine profiles and toll-like receptor (TLR) expression, all were capable of causing phenotypic and functional DC maturation. Both LPS and poly(I:C) downregulated TLR-4/MD-2 expression on DCs. Although poly(I:C) highly upregulated their cell surface TLR-9 expression, LPS upregulated the intracellular TLR-9 expression. LPS-treated DCs could not produce IL-12p70 in response to subsequent both LPS- and CpG DNA-stimulation. On the other hand, poly(I:C)-treated DCs retained to produce IL-12p70 by subsequent CpG DNA-stimulation, while subsequent LPS-stimulation did not induce IL-12p70 production. Chloroquine, inhibitor of endosomal maturation, completely inhibited cytokine production of LPS-treated DCs as well as unstimulated control in response to subsequent CpG DNA-stimulation, while it failed to delete the IL-12p40 and IL-10 production in poly(I:C)-treated DCs. These data suggest that poly(I:C) may induce a novel DC phenotype that preserves the capacity of cytokine production to subsequent CpG DNA-stimulation.     

Synergistic effect of Nod1 and Nod2 agonists with toll-like receptor agonists on human dendritic cells to generate interleukin-12 and T helper type 1 cells

A synthetic Nod2 agonist, muramyldipeptide (MDP), and two Nod1 agonists, FK565 and FK156, mimic the bacterial peptidoglycan moiety and are powerful adjuvants that induce cell-mediated immunity, especially delayed-type hypersensitivity. In this study, we used human dendritic cell (DC) cultures to examine possible T helper type 1 (Th1) responses induced by MDP and FK565/156 in combination with various synthetic Toll-like receptor (TLR) agonists, including synthetic lipid A (TLR4 agonist), the synthetic triacyl lipopeptide Pam3CSSNA (TLR2 agonist), poly(I:C) (TLR3 agonist), and CpG DNA (TLR9 agonist). Immature DCs derived from human monocytes expressed mRNAs for Nod1, Nod2, TLR2, TLR3, TLR4, and TLR9. The stimulation of DCs with MDP and FK565 in combination with lipid A, poly(I:C), and CpG DNA, but not with Pam3CSSNA, synergistically induced interleukin-12 (IL-12) p70 and gamma interferon (IFN-gamma), but not IL-18, in culture supernatants and induced IL-15 on the cell surface. In correlation with the cytokine induction, an upregulation of the mRNA expression of these cytokine genes was observed. Notably, IL-12 p35 mRNA expression increased >1,000-fold upon stimulation with lipid A plus either MDP or FK565 compared with stimulation with each stimulant alone. In contrast, for the expression of CD83 and costimulatory molecules such as CD40, CD80, and CD86, no synergistic effects were observed upon stimulation with Nod plus TLR agonists. The culture supernatants of DCs stimulated with lipid A plus either MDP or FK565 activated human T cells to produce high levels of IFN-gamma, and the activity was attributable to DC-derived IL-12. These findings suggest that Nod1 and Nod2 agonists in combination with TLR3, TLR4, and TLR9 agonists synergistically induce IL-12 and IFN-gamma production in DCs to induce Th1-lineage immune responses.     

Supernatant of Bifidobacterium breve induces dendritic cell maturation, activation, and survival through a Toll-like receptor 2 pathway

BACKGROUND: Commensal gut bacteria are essential for the development and maintenance of the gut's immune system. Some bacteria strains, such as Lactobacillus and Bifidobacterium species, have been reported to provide protection from allergic and inflammatory bowel diseases. However, the interactions between these commensal bacteria and the immune system are largely unknown. OBJECTIVE: We studied the effects of a supernatant from the culture of B breve C50 (BbC50) on the maturation, activation, and survival of human dendritic cells (DCs). METHODS: DCs were differentiated from human monocytes with IL-4 and GM-CSF for 5 days and cultured with BbC50 supernatant (BbC50SN) or LPS for 2 days. RESULTS: BbC50SN induced DC maturation, with increase in CD83, CD86, and HLA-DR expression. We also showed, for the first time, that BbC50SN prolonged DC survival, with high IL-10 and low IL-12 production compared with that seen in LPS-DCs. Moreover, BbC50SN inhibited the effects of LPS on DCs, both in terms of IL-12 production and in terms of survival. The prolonged DC survival was independent of IL-10 production and nuclear factor kappaB pathway but was associated with an upregulation of Bcl-xL and Phospho-Bad. Finally, BbC50SN induced activation of Toll-like receptor 2 (TLR2)-transfected cells in contrast to TLR4-, TLR7-, and TLR9-transfected cells. CONCLUSION: The supernatant of B breve C50 can induce DC maturation and prolonged DC survival through TLR2, with high IL-10 production. These properties might correspond to a regulatory DC profile, which could limit the excessive TH1 response and control the excessive TH2 polarization observed in atopic newborns.     

Role of MyD88 in diminished tumor necrosis factor alpha production by newborn mononuclear cells in response to lipopolysaccharide

Human newborns are more susceptible than adults to infection by gram-negative bacteria. We hypothesized that this susceptibility may be associated with a decreased response by leukocytes to lipopolysaccharide (LPS). In this study, we compared LPS-induced secretion of tumor necrosis factor alpha (TNF-alpha) by mononuclear cells (MNC) from adult peripheral blood and newborn umbilical cord blood in vitro and attempted to determine the mechanisms involved in its regulation. At a high concentration of LPS (10 ng/ml) and in the presence of autologous plasma, MNC from adults and newborns secreted similar amounts of TNF-alpha. However, in the absence of plasma, MNC from newborns secreted significantly less TNF-alpha compared to MNC from adults. Moreover, at a low concentration of LPS (0.1 ng/ml) and in the presence of plasma, TNF-alpha secretion was significantly lower for newborn MNC compared to adult MNC. Adults and newborns had similar numbers of CD14 and Toll-like receptor 4 (TLR-4)-positive cells as measured by flow cytometry. However, the intensity of the CD14 marker was greater for adult than for newborn cells. Incubation of cells with LPS led to an increase in CD14 and TLR-4 intensity for adult cells but not for newborn cells. The effect of LPS stimulation of adult or newborn cells was similar for ERK, p38, and IkappaBalpha phosphorylation, as well as IkappaBalpha degradation. Finally, we assessed levels of the TLR-4 adapter protein, the myeloid differentiation antigen 88 (MyD88). We found a direct relation between adult and newborn TNF-alpha secretion and MyD88, which was significantly decreased in newborn monocytes. Since TLR-4 signals intracellularly through the adapter protein, MyD88, we hypothesize that MyD88-dependent factors are responsible for delayed and decreased TNF-alpha secretion in newborn monocytes.     

Borrelia burgdorferi infection regulates CD1 expression in human cells and tissues via IL1-β

The appearance of group 1 CD1 proteins (CD1a, CD1b and CD1c) on maturing myeloid DC is a key event that converts myeloid DC to effective lipid APC. Here, we show that Borrelia burgdorferi, the causative agent of Lyme disease, triggers appearance of group 1 CD1 proteins at high density on the surface of human myeloid DC during infection. Within human skin, CD1b and CD1c expression was low or absent prior to infection, but increased significantly after experimental infections and in erythema migrans lesions from Lyme disease patients. The induction of CD1 was initiated by borrelial lipids acting through TLR-2 within minutes, but required 3 days for maximum effect. The delay in CD1 protein appearance involved a multi-step process whereby TLR-2 stimulated cells release soluble factors, which are sufficient to transfer the CD1-inducing effect in trans to other cells. Analysis of these soluble factors identified IL-1β as a previously unknown pathway leading to group 1 CD1 protein function. This study establishes that upregulation of group 1 CD1 proteins is an early event in B. burgdorferi infection and suggests a stepwise mechanism whereby bacterial cell walls, TLR activation and cytokine release cause DC precursors to express group 1 CD1 proteins.     

Suppression of dendritic cell activation by diabetes autoantigens linked to the cholera toxin B subunit

Antigen presenting cells, specifically dendritic cells (DCs) are a focal point in the delicate balance between T cell tolerance and immune responses contributing to the onset of type I diabetes (T1D). Weak adjuvant proteins like the cholera toxin B subunit when linked to autoantigens may sufficiently alter the balance of this initial immune response to suppress the development of autoimmunity. To assess adjuvant enhancement of autoantigen mediated immune suppression of Type 1 diabetes, we examined the cholera toxin B subunit (CTB)-proinsulin fusion protein (CTB-INS) activation of immature dendritic cells (iDC) at the earliest detectable stage of the human immune response. In this study, Incubation of human umbilical cord blood monocyte-derived immature DCs with CTB-INS autoantigen fusion protein increased the surface membrane expression of DC Toll-like receptor (TLR-2) while no significant upregulation in TLR-4 expression was detected. Inoculation of iDCs with CTB stimulated the biosynthesis of both CD86 and CD83 co-stimulatory factors demonstrating an immunostimulatory role for CTB in both DC activation and maturation. In contrast, incubation of iDCs with proinsulin partially suppressed CD86 co-stimulatory factor mediated DC activation, while incubation of iDCs with CTB-INS fusion protein completely suppressed iDC biosynthesis of both CD86 and CD83 costimulatory factors. The incubation of iDCs with increasing amounts of insulin did not increase the level of immune suppression but rather activated DC maturation by stimulating increased biosynthesis of both CD86 and CD83 costimulatory factors. Inoculation of iDCs with CTB-INS fusion protein dramatically increased secretion of the immunosuppressive cytokine IL-10 and suppressed synthesis of the pro-inflammatory cytokine IL12/23 p40 subunit protein suggesting that linkage of CTB to insulin (INS) may play an important role in mediating DC guidance of cognate naïve Th0 cell development into immunosuppressive T lymphocytes. Taken together, the experimental data suggests Toll like receptor 2 (TLR-2) plays a dominant role in CTB mediated INS inhibition of DC induced type 1 diabetes onset in human Type 1 diabetes autoimmunity. Further, fusion of CTB to the autoantigen was found to be essential for enhancement of immune suppression as co-delivery of CTB and insulin did not significantly inhibit DC costimulatory factor biosynthesis. The experimental data presented supports the hypotheses that adjuvant enhancement of autoantigen mediated suppression of islet beta cell inflammation is dependent on CTB stimulation of dendritic cell TLR2 receptor activation and co-processing of both CTB and the autoantigen in the same dendritic cell.     

The mast cell mediator PGD2 suppresses IL-12 release by dendritic cells leading to Th2 polarized immune responses in vivo

Dendritic cells (DC) and mast cells (MC) are colocalized in superficial organs such as the skin. Both cell types recognize and respond to pathogens. DC capture and transport antigens to the draining lymph node for CD4+ T cell priming and T helper 1 (Th1) or Th2 polarization. As MC are mainly associated with Th2 responses, DC-MC interactions may favor Th2 priming by DC. Here, we show the role of different MC mediators on IL-12 and IL-10 production by DC. While histamine, leukotriene C4, heparin and chondroitin sulfate A had little and unspecific effects on the cytokine production, prostaglandin D2 (PGD2) downregulated IL-10, IL-12p70 and p40. After subcutaneous (s.c.) injection of ovalbumin (OVA)-loaded, lipopolysaccharide (LPS)-matured DC into Th1-prone C57BL/6 mice, the levels of IFN-gamma produced by Th1 cells were decreased while IL-4 production remained low. When TNF-matured DC were pretreated with PGD2, loaded with the endotoxin-free antigen KLH and injected s.c. into Th2-prone BALB/c mice, we found a dose- and time-dependent upregulation of IL-4 and downregulation of IFN-gamma by T cells. Together, MC-derived PGD2 instructs DC to polarize CD4+ T cells towards Th2 responses. As a consequence, such a DC-MC cooperation may contribute to the maintenance of Th2 responses in allergic patients.